1. Field of Invention
The present invention relates to a power transmission system of a hybrid electric vehicle. More particularly, the present invention relates to a power transmission system of a hybrid electric vehicle which realizes at least two torque split modes and at least four fixed shift-speed modes.
2. Description of Related Art
Generally, a hybrid electric vehicle is a vehicle driven by at least two different types of power sources.
Such a hybrid electric vehicle typically includes an engine and a motor/generator. The hybrid electric vehicle uses a motor/generator having relatively better low-speed torque characteristics as a main power source at a low-speed and uses an engine having relatively better high-speed torque characteristics as a main power source at a high-speed.
Since the hybrid electric vehicle stops an operation of an engine using fossil fuel and uses a motor/generator at a low-speed region, fuel consumption may be improved and exhaust gas may be reduced.
The power transmission system of a hybrid electric vehicle is classified into a mono-mode type and a multi-mode type.
A torque delivery apparatus such as clutches and brakes for shift control is not necessary, but fuel consumption is high due to deterioration of efficiency at a high-speed region and an additional torque multiplication device is required for being applied to a large vehicle according to the mono-mode type.
Since the multi-mode type has high efficiency and is able to multiply torque autonomously, the multi-mode type can be applied to a full size vehicle.
Therefore, the multi-mode type instead of the mono-mode type is applied as the power transmission system of a hybrid electric vehicle and is also under continuous investigation.
The power transmission system of the multi-mode type includes a plurality of planetary gear sets, a plurality of motor/generators operated as a motor and/or a generator, a plurality of torque delivery apparatus controlling rotation elements of the planetary gear set, and a battery used as a power source of the motor/generators.
The power transmission system of the multi-mode type has different operating mechanisms depend on connections of the planetary gear sets, the motor/generators, and the torque delivery apparatus.
In addition, the power transmission system of the multi-mode type has different features such as durability, power delivery efficiency, and size depending on the connections of the planetary gear sets, the motor/generators, and the torque delivery apparatus. Therefore, designs for the connection structure of the power transmission system of a hybrid electric vehicle are also under continuous investigation.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.